Linux Command - grep

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grep command, which stands for “global regular expression print,” processes text line by line
and prints any lines which match a specified pattern.

To print only the matched part, use -o option

Options

command options

option description
-e PATTERN use PATTERN as the pattern. This can be used to specify multiple search patterns
-E, –Extended-regexp Interpret PATTERN as an extended regular expression
-i, --ignore-case ignore case in both the PATTERN and input files
-H print filename
-n, --line-number prefix each line of ouput with 1-based line number
-R, -r, --recursive read all files recursively
-v invert the matching
-o, –only-matching -o print only the matched (non-Empty) parts of a matching line

for context control

  • -A NUM –> print NUM lines of trailing output context
  • -B NUM –> print NUM lines of leading output context
  • -C NUM –> print NUM lines of output context

Example

find “foo” in file and prints lines that contains “foo”. Note that “foo”, “food” and “ofoo” are all matches.

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grep -i "foo" file1

find “foo” in files and prefix lines with filename and line number

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grep -inH "foo" words.txt

sample output:

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2
3
4
words.txt:1:food
words.txt:3:food
words.txt:4:foo
words.txt:7:ofoo

find exact match

The line matches exactly “foo”

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grep "^foo$"

Search the words that starts with ‘con’

search for words that start with con. examples are ‘contribution’, ‘control’. Only output the matched part

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cat LICENSE-2.0.txts | grep -o "con\w*"

here \w* is used to match any number of characters

Search the words that starts with ‘con’, then 3 characgers and ends with t. Here we need to use -E option to use extended regex.

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cat LICENSE-2.0.txt  | grep -o -E "con\w{3}t"

sample outputs are content and consist

Search directory recursively

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grep -inr "foo" dir

Output with context

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grep -in -C 3 "foo" filename

Search multiple patterns

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grep -in -E firefox -E chromium filename

Search pattern starts with hyphen(-)

Need to use -E option for searching String “-notify”

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grep -in -E -notify filename

ls directories in current directory

use regular expression to match the directories

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ls -l | grep "^d"

Similarly, to list regular files

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ls -l | grep "^-"

ls non-directories in current directory

use -v to invert the results.

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ls -l | grep -v "^d"

ls directories that ends with ‘k’

use regular expression to match the directories the ends with ‘k’

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ls -l | grep "^d.*k$"

Reference